Curbing Domestic Violence in Chickens

"Free-range" chickens are the gold standard for consumers interested in humanely raised livestock. But for most chickens, the wide-open spaces of a free-range poultry farm aren't nearly as idyllic as they sound. The birds often peck at each other's feathers, causing painful scars, bleeding, and even death. Now, researchers have developed a mathematical model that may help farmers stop the pecking before it starts.

It's unclear why chickens like to bite the feathers off their neighbors. According to bird-welfare researcher Bas Rodenburg of Wageningen University in the Netherlands, the best explanation is that they've evolved to peck for food in the wild, and this need is not satisfied on the farm. "Instead of pecking at the floor, for instance, they start pecking at each other's feathers," Rodenburg says. Right now, the only way for free-range farmers to prevent the behavior is beak trimming, a euphemism for cutting off the sharp tip of a bird's beak with a hot blade or directing infrared rays into its inner tissue until the tip falls off a few weeks later.

To find a better solution, a team of zoologists and engineers studied video recordings of more than 300,000 hens living on free-range farms in the United Kingdom. The researchers applied a mathematical technique called optical flow modeling, which has been used to study traffic patterns and human crowds, to track how the chickens moved in large groups. The process involved analyzing multiple snapshots of the same 50 to 100 hens taken at different times to find patterns of movement that correlate with chicken-on-chicken violence.

Over time, a distinct pattern emerged. Chickens that moved around frequently tended not to peck at their companions. But birds that sat still for long periods of time and then showed sudden flurries of motion were more likely to hurt each other, the team reported online this month in the Journal of the Royal Society Interface. Factors such as diet, breed, and air quality also influenced which groups were more prone to pecking.

Co-author Marian Dawkins of the University of Oxford in the United Kingdom says the statistical model could anticipate with 87% accuracy which flocks of hens were most likely to become chronic feather-biters in the future. The team could make predictions as far in advance as 17 to 20 weeks old—just after the birds are transported to the laying farm, though it got more difficult the further ahead they tried to look. Although it's just a preliminary study, Dawkins hopes the model may one day help farmers decide which chickens to keep together and which to separate. And that could result in a free-range farm that's truly humane for chickens.

"For practical purposes, I think this could be an interesting approach," says Rodenburg, who was not part of the team. Still, he says he would prefer a model that could predict feather damage even further in advance. "What you would really like, of course, is to mark flocks of hens that are at risk to develop feather pecking as early as possible, so preferably before they even move to the laying farm."

For egg farmers in Europe at least, the solution can't come soon enough. Starting in 2012, the European Union will ban conventional cages—which often prevent the chickens from moving around, spreading their wings, or even standing up straight—for laying hens. Enriched cages, which give the hens slightly more space and include nest boxes and perches, will still be allowed under the new rules. But some E.U. countries are looking to ban those as well, which means the more vicious birds will have to learn to live with each other.